John F. Ferguson

Active displacement transfer and differential block motion within the central Walker Lane, western Great Basin

Velocities determined for 50 global positioning system sites within the central Walker Lane indicate differential motion among tectonic blocks forming a boundary zone between the Great Basin extensional province and the Sierra Nevada. The velocity field is related to displacement transfer from the Owens Valley and Furnace Creek fault systems of east- ern California to transtensional structures of the Walker Lane and extensional faults of the central Nevada seismic belt. Block boundaries are sharp and appear to be inherited from pre-Tertiary crustal structure. The block geometries exert strong influence on dif- ferential displacements concentrated along boundaries as belts of divergent, transcurrent, and convergent motion. The aggregate velocity accounts for about 25% of the relative motion between the Pacific and North American plates. About 5 mm/yr of the motion is localized along the eastern margin of the Sierra Nevada, whereas about 10 mm/yr is stepped 100 km east along a belt of east-northeast-trending transtensional faults that merge with northwest-trending transcurrent structures of the Walker Lane. About 6 mm/ yr of the velocity field is transferred to north-northeast-trending extensional faults of the central Nevada seismic belt. The heterogeneous distribution of motions is consistent with partitioning of a regional velocity field formed by westward extension and N408W-directed shear.

Outcrop fracture characterization using terrestrial laser scanners: Deep-water Jackfork sandstone at Big Rock Quarry, Arkansas

Determination of fracture orientation can be an important aspect of structural analysis in reservoir characterization. The availability of ground-based laser scanner systems opens up new possibilities for the determination of fracture surface orientation in rock outcrops. Scanners are available in low-sample-density, low-accuracy, and fast, high-sample-density, high-accuracy models. These automatic laser scanner systems produce enormous volumes or “clouds” of point data at an instrument-dependent accuracy and resolution, which can be at the millimeter level. This huge volume of data calls for an automated and objective method of analysis. We have developed a surface classification algorithm based on a multipass partitioning of the point cloud. The method makes use of both spatial proximity and the orientation of an initial coarse-grained model of the point cloud. Unsupervised classification of surface sets is demonstrated herein using the new algorithm. Both previously mentioned types of scanners have been used to map the Jackfork sandstone outcrop at Big Rock Quarry in Little Rock, Arkansas. We apply the surface classification algorithm to these data to extract fracture surface orientations from the point cloud. The effectiveness of these new technologies when applied to fracture analysis is clearly demonstrated in this example. It is also shown that the low-density, low-resolution type of scanner is adequate to define general geomorphology but is inadequate for fracture definition. The surface classification algorithm can be used to reliably extract fracture and bedding strike and dip angles from the three-dimensional point locations acquired using centimeter-accurate, high-density laser scanner systems.

The 4-D microgravity method for waterflood surveillance; a model study for the Prudhoe Bay reservoir, Alaska

Forward and inverse gravity modeling is carried out on a suite of reservoir simulations of a proposed water injection in the Prudhoe Bay reservoir, Alaska. A novel surveillance technique is developed in which surface gravity observations are used to monitor the progress of a gas cap waterflood in the reservoir at 8200-ft (2500-m) depth. This cost‐effective method requires that high‐precision gravity surveys be repeated over periods of years. Differences in the gravity field with time reflect changes in the reservoir fluid densities. Preliminary field tests at Prudhoe Bay indicates survey accuracy of 5–10 μGal can be achieved for gravity data using a modified Lacoste & Romberg “G” type meter or Scintrex CG-3M combined with the NAVSTAR Global Positioning System (GPS). Forward gravity modeling predicts variations in surface measurements of 100 μGal after 5 years of water injection, and 180–250 μGal after 15 years. We use a constrained least‐squares method to invert synthetic gravity data for subsurface densi...

The 4D microgravity method for waterflood surveillance: Part II — Gravity measurements for the Prudhoe Bay reservoir, Alaska

Between 1994 and 2002, a series of experiments was conducted at Prudhoe Bay, Alaska, aimed at the development of an effective 4D (or time-lapse) gravity technique. Theoretical investigations had pointed out the potential for monitoring water injection in the 2500-m -deep reservoir, but it was not clear that gravity measurements of sufficient accuracy could be made in the arctic environment. During the course of these experiments, new techniques and instrumentation were introduced and perfected for both gravity and position measurements. Gravity stations are located using high-precision global positioning system (GPS) techniques without permanent monuments. Robust methods for meter drift control have improved noise resistance in relative gravimeter surveys. Absolute gravity measurements with a field-portable instrument maintain absolute gravity levels among surveys. A 4D gravity-difference noise of 12 μGal standard deviation has been established at Prudhoe Bay for GPS-controlled relative gravimeter surveys...

Regularized two-dimensional Fourier gravity inversion method with application to the Silent Canyon Caldera, Nevada

A modification of the 2‐D Fourier gravity inversion method includes regularization and a linear density variation with depth. Explicit downward continuation in the Fourier inversion of gravity observations from mass distributions at depth produces instability in the presence of noise and shallow mass distributions. A data‐adaptive regularization filter tapers growth of the exponential continuation function. An empirical relationship between the regularization filter parameter and a parametric model of potential field spectra results in automatic selection of the filter parameter for a given continuation depth. Inversion of synthetic data from a random noise‐contaminated basin type model produces a depth model that agrees with the synthetic structure with an rms error commensurate with the data noise. A model of the Silent Canyon caldera, buried beneath Pahute Mesa at the Nevada Test Site, results in a gravity field that agrees with the observations to within a 4 percent rms error. The caldera gravity mode...

The 4D microgravity method for waterflood surveillance: Part 3 - 4D absolute microgravity surveys at Prudhoe Bay, Alaska

The 4D microgravity method is becoming a mature technology.Aprojecttodeveloppracticalmeasurementandinterpretation techniques was conducted at Prudhoe Bay,Alaska, from1994through2002.Beginningin2003thesetechniques have been systematically applied to monitor a waterflood in the gas cap of the Prudhoe Bay reservoir. Approximately 300 stations in a 150 km 2 area are reoccupied in each survey year with sub-5 Gal precision absolute gravity and centimeter precision Global Positioning System GPS geodetic measurements. The 4D gravity measured over epochs 2005‐2003, 2006‐2003, and 2007‐2003 has been successfullymodeledtotrackthemassofwaterinjectedsincelatein 2002.AnewandimprovedversionoftheA-10field-portable absolute gravity meter was developed in conjunction with this project and has proven to be a key element in the success of the 4D methodology.The use of an absolute gravity meter in a field survey of this magnitude is unprecedented. There aresubstantialdifferencesbetweena4Dabsolutemicrogravity survey and a conventional gravity survey in terms of station occupation procedures, GPS techniques, and the 4D elevation correction. We estimate that the overall precision of the4Dgravitysignalineachepochislessthan10 Gal.

Unique spinel-garnet lherzolite inclusion in kimberlite from Australia

An upper-mantle nodule found in kimberlite of Cenozoic age in southeastern Australia contains the assemblage aluminous spinel + garnet + orthopyroxene + clinopyroxene + olivine. This inclusion is the first recorded occurrence of an equilibrium mineral assemblage lying on the quasi-univariant boundary separating the spinel and garnet lherzolite fields inferred in the upper mantle. The temperature and pressure estimate of 1240 ± 20 °C and 22 ± 1 kb for the source of the inclusion indicates an abnormally high geothermal gradient, actually exceeding die oceanic geotherm. Heat-flow measurements also indicate a similarly high geothermal gradient for this part of Australia. It is inferred that this nodule is derived from a region of perturbed steady-state geotherm, generated during Tertiary volcanism and continuing to the present day.

A geophysical‐geological transect of the Silent Canyon Caldera Complex, Pahute Mesa, Nevada

Revision of lithological logs for boreholes penetrating the volcanic center at Pahute Mesa, Nevada, has led to a thorough review of the volcanic stratigraphy and geologic structure. We have combined this review with a compilation of old and newly acquired gravity and seismic travel time data, producing a unified interpretation along a northwest to southeast profile. The analysis supports a new interpretation of the Silent Canyon caldera complex. The caldera is found to be more asymmetric than previously suggested, with the southeastern boundary formed by linear, high-angle normal faults and a more gently sloping northwestern boundary. The total thickness of volcanic units within the caldera complex does not appear to exceed 5 km. The shallow structure at Pahute Mesa could have a profound effect on the seismic response for regional and teleseismic signals from this nuclear test site. The Silent Canyon caldera complex is actually a set of nested calderas first filled by thick (>1 km) postcaldera lavas and subsequently buried by outflow sheets of the Timber Mountain caldera to the south. Thick, postcaldera lavas filled a half-graben structure formed west of the West Greeley fault, dropping the tops of the youngest caldera-forming units to depths in excess of 2 km. Therefore the western boundary of the caldera complex is poorly defined. East of the West Greeley fault, two overlapping calderas are defined, and stratigraphic data suggest the presence of even older calderas. The youngest caldera, the calc-alkaline Area 20 caldera, is well defined from drill hole data. The Area 20 caldera overlaps the 13.6 Ma peralkaline Grouse Canyon caldera, which is less well defined, but apparently collapsed in trap-door style along the Almendro fault. For both these calderas, collapse continued after the main caldera-forming eruption, concurrent with the accumulation of thick (>1 km) lavas within the peripheral collapse zones. The geophysical interpretation indicates that the major structural boundary of the caldera complex corresponds to the NNE trending Scrugham Peak and Almendro faults, which offset the pre-Tertiary contact more than 1 km but have less than 200 m offset in rocks of 11 Ma age. Drill hole data show that offsets along these faults increase systematically within older (up to 15 Ma) units, which are commonly rotated eastward in a style similar to units at the surface. Abrupt changes in the subsurface thickness of the caldera-forming units occur across the faults, indicating that these linear features served as caldera boundaries.

Models of the Bouguer gravity and geologic structure at Yucca Flat, Nevada

The Bouguer gravity anomaly at Yucca Flat, Nevada has been modeled by two different techniques: the Cordell-Henderson and Parker-Oldenburg methods. The three-dimensional model has incorporated known density and structural information where possible. These models predict the structural relief on the Cenozoic-Paleozoic contact to within 150 m or about 15 percent of the actual depth.The three-dimensional Parker-Oldenburg method has been found to be efficient in an application involving a large (9000 sample) data base. Numerical stability was ensured by the application of a consistent regularization (a low-pass filter tuned to suppress the noise-dominated portion of the data spectrum) of the downward continuation operator. The use of a single regularizing filter for the entire model is not completely satisfactory due to the oversmoothing of shallow regions of the basin.The model is useful in the delineation of the geologic history of the area. Structural features in the model support the hypothesis that regional stress fields rotated significantly during the Tertiary. Major structural elements of the basin are well defined on the Cenozoic-Paleozoic interface. The principal basin-bounding fault is the large-throw Carpetbag fault on the west. This fault was most active during the earliest phases of subsidence. The Yucca fault is seen to be a much smaller feature in the model presented here. The basin is rotated down to the west, with normal hinge faults on the eastern margin.

The 4D microgravity method for waterflood surveillance: Part IV — Modeling and interpretation of early epoch 4D gravity surveys at Prudhoe Bay, Alaska

Between March 2003 and March 2007, four high-precision 4D absolute microgravity surveys were performed at Prudhoe Bay, Alaska. These surveys are part of an ongoing effort to monitor the progress of a very large water-injection project in the gas cap of the Prudhoe Bay reservoir at a depth of 2.5 km . These carefully acquired gravity data must be modeled and interpreted in terms of water movement within the reservoir. A constrained linear inversion scheme was tested on reservoir simulations during the planning and development phase of this project (preinjection). The inver-sion methodology has been applied to data for three epochs (2005–2003, 2006–2003, and 2007–2003), and mass-distribution models have been produced for the reservoir. The time evolution of the water-mass distribution in the reservoir is visualized from these three snapshot models. The waterflood is expanding into the gas cap at the expected rate but is exhibiting nonsymmetric behavior that is consistent with a greater degree of structural ...